There has recently been growing a demand for a weldable black steel sheet having an excellent appearance as a material used for electrical appliances such as video tape recorders and copying machines, etc. A wide variety of attempts have, therefore, been made to develop methods for the blackening treatment of a steel sheet. Moreover, there has been growing a demand for a low-gloss black steel sheet which can satisfy the consumers' desire for an article for appearance of low gloss and apparently of high quality, and also the requirement that no clearly visible fingerprint be left on the surface of a black steel sheet by anybody touching it during its shaping, or its use in the fabrication of a particular article.
The following in an summary of the methods which have usually been employed for forming a black film on a steel sheet:
(a) A black paint containing a black pigment, such as carbon black, is applied onto a steel sheet by e.g. spraying or roll coating to form thereon a film having a thickness of several tens of microns. PA1 (b) Metal coatings plated on steel sheets are reacted or electrolized in various solutions to form black films. This method includes a number of modes as will hereunder be set forth: PA1 (c) A treating solution obtained by adding an organic dye to an aqueous solution of potassium sodium silicate is applied onto a surface plated with zinc or an alloy (Japanese Patent Publication No. 30593/1980). PA1 (a) This is a common method of coating used to form an outer surface coating, the thickness of which is usually at least 10 microns when it consists of a single layer. This thickness is too large to allow the welding of the steel sheet. It is however, difficult to obtain a excellent black appearance of black film having a thin thickness allowing the welding of the sheet, even if the solution may contain the black pigment, such as carbon black, at the maximum possible concentration. In other words, the use of any such blackening agent has been found unable to form any thin black coated steel sheet that has both good black appearance and weldability. PA1 (b) (1) The solution containing Ag ions is expensive. Moreover, this method spends as long a time as several tens of seconds in forming a black film and is, therefore, inapplicable to the continuous treatment of a strip of steel sheet which need be finished as rapidly as within five seconds. PA1 (2) The black film which is formed by this method is unsatisfactory in formability. Insofar as carbon black is a conductive pigment, the film is electrically conductive and is, therefore, low in corrosion resistance. Moreover, the film is unsatisfactory in blackness. PA1 (3) and (4) These methods are both uneconomical, since they rely upon the dissolution of metal from the plated layer on the steel sheet. Moreover, the metal ions as dissolved from the plated layer deteriorate the solution for the blackening treatment and can present a serious problem to the continuous treatment. Furthermore, both of the methods have only a limited scope of application, i.e. (3) is applicable only to a steel sheet plated with a Zn-Co, Zn-Ni or Zn-Mo alloy, while (4) is applicable only to a sheet plated with a Zn-Ni alloy. PA1 (5) The black film which is formed by this method is unsatisfactory in formability. PA1 (6) The black film which is formed by this method is unsatisfactory in adhesive strength, and is also low in corrosion resistance, insofar as a metal having a noble or high potential is deposited on a metal having a base or low potential (i.e. on zinc or an alloy thereof). PA1 (c) This method is not intended for forming a film having an excellently black surface, nor is it intended for imparting weldability, insofar as the disclosure does not contain any specific reference to the film thickness. Moreover, as the film is basically composed of potassium sodium silicate, the hardened film is unsatisfactory in lubricating property as required during press forming, and is, therefore, unsuitable for any steel sheet that is used for making domestic electrical appliances, office machines or furniture, etc. PA1 (1) A weldable low-gloss black steel sheet which comprises a steel sheet plated with zinc or a zinc alloy, and carrying a chromate film formed on its surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a black dye as a blackening agent and 1 to 150 parts by weight of organic resin particles, the thermosetting resin and the dye being soluble in an organic solvent. PA1 (2) A weldable low-gloss black steel sheet which comprises a steel sheet plated with zinc or a zinc alloy, and carrying a chromate film formed on its surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a black dye as a blackening agent, 1 to 150 parts by weight of organic resin particles and 1 to 100 parts by weight of fine inorganic particles, the thermosetting resin and the dye being soluble in an organic solvent. PA1 (3) A weldable low-gloss black steel sheet which comprises a steel sheet plated with zinc or a zinc alloy, and carrying a chromate film formed on its surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a black dye as a blackening agent, 1 to 150 parts by weight of organic resin particles and 1 to 100 parts by weight of a solid lubricant, the thermosetting resin and the dye being soluble in an organic solvent. PA1 (4) A weldable low-gloss black steel sheet which comprises a steel sheet plated with zinc or a zinc alloy, and carrying a chromate film formed on its surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a black dye as a blackening agent, 1 to 150 parts by weight of organic resin particles, 1 to 100 parts by weight of fine-inorganic particles and 1 to 100 parts by weight of a solid lubricant, the thermosetting resin and the dye being soluble in an organic solvent. PA1 (5) A weldable low-gloss black steel sheet as set forth at (2) or (4) above, wherein the fine inorganic particles are of one or more materials selected from among silica, an extender pigment, a sparingly soluble chromic acid salt which has low solubility in water and carbon black. PA1 (6) A weldable low-gloss black steel sheet as set forth at (3) or (4) above, wherein the black film contains as the solid lubricant one or more substances selected from among polyolefin wax, paraffin wax, fluororesins, fatty acid amides, metallic soaps, molybdenum disulfide, graphite, graphite fluoride, boron nitride and polyalkylene glycols. PA1 (7) A weldable low-gross black steel sheet which comprises a steel sheet plated with zinc or a zinc alloy, and carrying a chromate film formed on its surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a black dye as a blackening agent and 10 to 150 parts by weight of organic resin particles having an average diameter of 1 to 25 microns, the thermosetting resin and the dye being soluble in an organic solvent, the black film having a thickness of 0.3 to 5.0 microns. PA1 (8) A weldable low-gross black steel sheet which comprises a steel sheet plated with zinc or a zinc alloy, and carrying a chromate film formed on its surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on he chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a black dye as a blackening agent, 10 to 150 parts by weight of organic resin particles having an average diameter of 1 to 25 microns, and 1 to 30 parts by weight of silica such as a precipitated type or a gel type, obtained by the reaction of sodium silicate and mineral acids, the thermosetting resin and the dye being soluble in an organic solvent, the black film having a thickness of 0.3 to 5.0 microns. PA1 (9) A weldable low-gloss black steel sheet which comprises a steel sheet plated with zinc or a zinc alloy, and carrying a chromate film formed on its surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a black dye as a blackening agent, 10 to 150 parts by weight of organic resin particles having an average diameter of 1 to 25 microns and 1 to 40 parts by weight of particles of a fluorine compound, the thermosetting resin and the dye being soluble in an organic solvent, the black film having a thickness of 0.3 to 5.0 microns. PA1 (10) A weldable low-gloss black steel sheet which comprises a steel sheet plated with zinc or a zinc alloy, and carrying a chromate film formed on its surface and having a coating weight of 1 to 200 mg/m.sup.2 in terms of metallic chromium, and a black film formed on the chromate film from a composition comprising 100 parts by weight of a thermosetting resin as a base resin, 1 to 200 parts by weight of a black dye as a blackening agent, 10 to 150 parts by weight of organic resin particles having an average diameter of 1 to 25 microns, 1 to 30 parts by weight of particles by weight of silica such as a precipitated type or a gel type, obtained by the reaction of sodium silicate and mineral acids, and 1 to 40 parts by weight of particles of a fluorine compound, the thermosetting resin and the dye being soluble in an organic solvent, the black film having a thickness of 0.3 to 5.0 microns. PA1 (11) A weldable low-gloss black steel sheet as set forth at any of ( 7 ) to ( 10 ) above, wherein the black film further contains a total of 1 to 40 parts by weight of at least one of a sparingly soluble chromic acid salt and fumed silica produced by pyrogenic processes. PA1 (12) A weldable low-gloss black steel sheet as set forth at any of ( 7 ) to ( 10 ) above, wherein the black film further contains a total of 1 to 40 parts by weight of at least one of polyethylene wax and paraffin wax. PA1 (13) A weldable low-gloss black steel sheet as set forth at any of (7) to (10) above, wherein the black film further contains ( A ) and ( B ): PA1 (14) A weldable low-gloss black steel sheet as set forth at any of (1) to (13) above, wherein the organic resin particles are of one or more resins selected from among urethane, silicone, epoxy, amino, acrylic, acrylonitrile, acryl-urethane, polyamide, polyester and polypropylene resins. PA1 (15) A weldable low-gloss black steel sheet as set forth at any of (1) to (13) above, wherein the organic resin particles are black particles of one or more resins selected from among urethane, silicone, epoxy, amino, acrylic, acrylonitrile, acryl-urethane, polyamide, polyester and polypropylene resins. PA1 (16) A weldable low-luster black steel sheet as set forth at any of (1) to (13) above, wherein the black film contains as the black dye one or more dyes selected from among azo and azine dyes. PA1 (17) A weldable low-luster black steel sheet as set forth at any of (1) to (13) above, wherein the black dye is an metal complex of azo dye. PA1 (18) A weldable low-luster black steel sheet as set forth at any of (1) to (13) above, wherein the black dye is a mixture of an metal complex of azo dye and a phthalocyanine dye. PA1 (19) A weldable low-gloss black steel sheet as set forth at any of (1) to (18) above, wherein the sheet carries the back film only on one side thereof, while on the other side thereof, it carries a chromate film having a coating weight of 10 to 120 mg/m.sup.2 in terms of metallic chromium. PA1 (20) A weldable low-gloss black steel sheet as set forth at (19) above, wherein the chromate film on the other side of the sheet is a film formed by applying a chromating solution containing hexavalent and trivalent chromium ions in a weight ratio of 40/60 to 80/20 (hexavalent/trivalent) in terms of metallic chromium, whereafter it is heated and dried. PA1 (21) A weldable low-gloss black steel sheet as set forth at (19) above, wherein the chromate film on the other side of the sheet is a film formed by applying a chromating solution containing hexavalent and trivalent chromium ions and a colloidal silica, the hexavalent and trivalent chromium ions having a weight ratio of 40/60 to 80/20 (hexavalent/trivalent) in terms of metallic chromium, and the colloidal silica and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/2 to 8/1 (colloidal silica/chromium ions), whereafter it is heated and dried. PA1 (22) A weldable low-gloss black steel sheet as set forth at (19) above, wherein the chromate film on the other side of the sheet is a film formed by applying a chromating solution containing hexavalent and trivalent chromium ions and an aqueous resin, the hexavalent and trivalent chromium ions having a weight ratio of 40/60 to 80/20 (hexavalent/trivalent) in terms of metallic chromium, and the aqueous resin and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/20 to 40/1 (aqueous resin/chromium ions), whereafter it is heated and dried. PA1 (23) A weldable low-gloss black steel sheet as set forth at (19) above, wherein the chromate film on the other side of the sheet is a film formed by applying a chromating solution containing hexavalent and trivalent chromium ions, a colloidal silica and an aqueous resin, the hexavalent and trivalent chromium ions having a weight ratio of 40/60 to 80/20 (hexavalent/trivalent) in terms of metallic chromium, and the colloidal silica and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/2 to 8/1 (collidal silica/chromium ions), and the aqueous resin and the total of chromium ions in terms of metallic chromium having a weight ratio of 1/20 to 40/1 (aqueous resin/chromium ions), whereafter it is heated and dried. PA1 (24) A weldable low-gloss black steel sheet as set forth at (19) above, wherein the chromate film on the other side of the sheet is a film formed by applying a chromating solution containing hexavalent and trivalent chromium ions trivalent PO.sub.4 ions and divalent zinc ions, the hexavalent and trivalent chromium ions having a weight ratio of 40/60 to 80/20 (hexavalent/trivalent) in terms of metallic chromium, the trivalent PO.sub.4 ions and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/30 to 2/1 (PO.sub.4 ions/chromium ions), and the divalent zinc and the hexavalent chromium ions in terms of metallic chromium having a weight ratio of 1/40 to 2/3 (zinc ions/chromium ions), whereafter it is heated and dried. PA1 (25) A weldable low-gloss black steel sheet as set forth at (19) above, wherein the chromate film on the other side of the sheet is a film formed by applying a chromating solution containing hexavalent and trivalent chromium ions, a colloidal silica, trivalent PO.sub.4 ions and divalent zinc ions, the hexavalent and trivalent chromium ions having a weight ratio of 40/60 to 80/20 (hexavalent/trivalent) in terms of metallic chromium, the (colloidal silica and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/2 to 8/1 (colloidal silica/chromium ions), the trivalent PO.sub.4 ions and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/30 to 2/1 (PO.sub.4 ions/chromium ions), and the divalent zinc ions and the hexavalent chromium ions in terms of metallic chromium having a weight ratio of 1/40 to 2/3 (zinc ions/chromium ions), whereafter it is heated and dried. PA1 (26) A weldable low-gloss black steel sheet as set forth at (19) above, wherein the chromate film on the other side of the sheet is a film formed by applying a chromating solution containing hexavalent and trivalent chromium ions, an aqueous resin, trivalent PO.sub.4 ions and divalent zinc ions, the hexavalent and trivalent chromium ions having a weight ratio of 40/60 to 80/20 (hexavalent/trivalent) in terms of metallic chromium, the aqueous resin and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/20 to 40/1 (aqueous resin/chromium ions), the trivalent PO.sub.4 ions and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/30 to 2/1 (PO.sub.4 ions/chromium ions), and the divalent zinc ions and the hexavalent chromium ions in terms of metallic chromium having a weight ratio of 1/40 to 2/3 (zinc ions/chromium ions), whereafter it is heated and dried. PA1 (27) A weldable low-gloss black steel sheet as set forth at (19) above, wherein the chromate film on the other side of the sheet is a film formed by applying a chromating solution containing hexavalent and trivalent chromium ions, a colloidal silica, an aqueous resin, trivalent PO.sub.4 ions and divalent zinc ions, the hexavalent and trivalent chromium ions having a weight ratio of 40/60 to 80/20 (hexavalent/trivalent) in terms of metallic chromium, the colloidal silica and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/20 to 8/1 (colloidal silica/chromium ions), the aqueous resin and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/20 to 40/1 aqueous resin/chromium ions), the trivalent PO.sub.4 ions and the total of the chromium ions in terms of metallic chromium having a weight ratio of 1/30 to 2/1 (PO.sub.4 ions/chromium ions), and the divalent zinc ions and the hexavalent chromium ions in terms of metallic chromium having a weight ratio of 1/40 to 2/3 (zinc ions/chromium ions), whereafter it is heated and dried.
(1) A chromating solution containing Ag ions is used to form a black chromate film (as proposed in Japanese Patent Application Laid-Open No. 193376/1983); PA2 (2) A black resin film composed mainly of carbon black is electro deposited (Japanese Patent Application Laid-Open No. 62996/1981); PA2 (3) A steel sheet which has been electroplated with a Zn-Co, Zn-Ni or Zn-Mo alloy is subjected to anodizing (Japanese Patent Publication No. 38276/1986); PA2 (4) A steel sheet which has been plated with a Zn-Ni alloy is subjected to immersing, spraying or anodizing with a solution containing nitric acid or a nitrate group to develop a black surface (Japanese Patent Publication No. 30262/1987); PA2 (5) A black plated film formed by cathodic treatment (e.g. Japanese Patent Application Laid-Open No. 263995/1987); and PA2 (6) Displacement plating is carried out on a surface plated with zinc or an alloy thereof to deposit thereon a metal having a nobler potential (e.g. Japanese Patent Application Laid-Open No. 89879/1987). PA2 (A) a total of 1 to 40 parts by weight of at least one of a sparingly soluble chromic acid salt and fumed silica; and PA2 (B) a total of 1 to 40 parts by weight of at least one of polyethylene wax and paraffin wax.
All of these methods do, however, have their own drawbacks, as will hereunder be pointed out.
Moreover, none of the above methods is intended for forming a low-gloss, or matted black film, but the film formed by any of those methods has a distinctly lustrous surface.